This invention is related to digital printing generally, and is more specifically related to a transfer printing process using an ink which is curable by ultraviolet radiation.
Transfer processes involve physically transferring an image from one substrate to another and can be achieved in several ways. One method is melt transfer printing where a design is first printed on paper using a waxy ink. Melt transfer printing has been used since the nineteenth century to transfer embroidery designs to fabric. A design is printed on paper using a waxy ink, then transferred with heat and pressure to a final substrate. The Star process, developed by Star Stampa Artistici di Milano, uses a paper that is coated with waxes and dispersing agents. The design is printed onto the coated paper by a gravure printing process using an oil and wax based ink. The print is then transferred to fabric by pressing the composite between heated calendar rollers at high pressure. The ink melts onto the final substrate carrying the coloring materials with it. Fabrics printed in such a method using direct dyes are then nip-padded with a salt solution and steamed. Vat dyes can also be used in the ink, but the fabric must be impregnated with sodium hydroxide and hydrous solution and steamed. The residual waxes from the transfer ink are removed during washing of the fabric.
Another method of transfer printing is film release transfer. Here the image is printed onto a paper substrate coated with a film of heat tackifiable resin. Upon application of heat and pressure to the backside of the image, the entire film containing the image is transferred to the final substrate. A process of thermal transfer wherein the ink physically bonds to the substrate is described in, for example, Hare, U.S. Pat. No. 4,773,953. The resulting image, as transferred, is a surface bonded image with a raised, plastic-like feel to the touch. Thermal transfer paper can transfer an image to a final substrate, such as cotton, however, this method has several limitations. First, the entire sheet is transferred, not just the image. Second, such papers are heavily coated with polymeric material to bind the image onto the textile. This material makes the transfer area very stiff and has poor dimensional stability when stretched.
Another method of transfer employs the use of heat activated, or sublimation, dyes. One form of an appropriate transfer process using sublimation inks is described in Hale, et. al., U.S. Pat. No. 5,601,023, the teachings of which are incorporated herein by reference. In this situation, an image is printed onto an intermediate medium, such as paper, followed by application of heat and pressure to the backside of the intermediate medium while in contact with a final substrate. The dyes then vaporize and are preferentially absorbed by the final substrate. Sublimation dyes yield excellent results when a polyester substrate is used and are highly resistant to fading and abrasion caused by laundering. These dyes, however, have a limited affinity for substrates other than polyester, and give poor results on natural fibers such as cotton and wool.
A method of preparing an image-receiving sheet for sublimation transfer utilizing isocyanate groups is described in DeVries, U.S. Pat. No. 4,058,644. Here, a polyisocyanate is reacted with a polyol to form a polyurethane containing free or blocked isocyanate groups. A print paste containing this polymer along with a sublimation dye is coated onto a paper substrate via roller coating, brush coating, silk screening, or similar method. The image may then be heat transferred to a cotton substrate. The application of heat to the backside of the printed image activates the sublimation dye as well as the blocked isocyanate groups. The isocyanate groups become unblocked at the transfer temperature and available to react with hydroxyl groups on the cellulose fibers, therefore forming a chemical bond with the cellulose fiber.
One method of transferring only a portion of a printed image is described in Sideman, et. al., U.S. Pat. No. 4,619,665. Here a sheet is first layered with a mixture of reactive and non-reactive sublimation dyes. Polyethyleneimines, which react with the reactive sublimation dyes, are then deposited on the dyes in a pattern via conventional printing techniques such as gravure or flexographic printing. The structure and the substrate to be printed are placed in contact with each other, and upon heating, only the dyes in the areas not printed with the polyethyleneimines and the non-reactive dyes under or mixed with the thus printed polyethyleneimines are sublimed to the final substrate. The reactive sublimation dyes are blocked from sublimating by reaction with the polyethyleneimines.
The use of photopolymerizable materials to form durable coatings is well known. Photochemically produced polymerization reactions have become increasingly important for rapid curing of thin films in such areas as the curing of paint and plastic coatings on paper, metal and wood, and in the drying of printing inks. The photopolymerizable coating is a coating that is deposited on a substrate, and subsequently cured to form a final coating that is firmly affixed to the substrate. The curing step is performed by exposure of the coating to some form of radiation. In a typical application, UV radiation is the method of curing. UV radiation is sufficiently energetic to initiate certain chemical reactions when a photoinitiator is present, but electromagnetic radiation more energetic may be used. In addition, exposure to electron beams may be used to initiate polymerization of a coating.
The process of photopolymerization involves first absorption of incident radiation by an absorbing molecule within the chemical system. The absorbing molecule may be a photosensitizer or a photoinitiator. A photosensitizer is not consumed in subsequent chemical reactions, but rather acts as a photocatalyst by transferring internal energy to another molecule, which then initiates the polymerization reaction. By transferring its internal energy, the photosensitizer is returned to its original state. A photoinitiator is consumed in the polymerization reaction. The photoinitiator is excited, ionized or fragmented by incident radiation, then initiates polymerization. xe2x80x9cPhotoinitiatorxe2x80x9d can, however, be used in general terms to mean any species which interacts with the incident radiation.
A coating may contain monomers and/or oligomers, along with the photoinitiators and other additives. After application to the substrate, the curing step involves polymerization of the coating to form the final cured coating on the substrate. UV curable coatings applied as a liquid are typically solutions of monomers. Coatings applied as solids are frequently partially polymerized prior to application to the substrate. The partially polymerized polymers are known as oligomers or prepolymers. The curing of the solid coating on the substrate then involves polymerization of the remaining functionality to form the final coating. Often a coating formulation will comprise a mixture of both monomers and oligomers.
There are two general classes of polymerization of primary concern, cationic and free radical systems. In cationic cure systems, positively charged species are the primary mechanism for curing the polymer. Epoxy resins are the most common chemical species used for this type of cure. In free radical systems, a free radical is the chemical species responsible for the curing reactions. Common resins of this type include acrylates, unsaturated polyesters, polyene/thiol systems, maleates and vinyl/acrylics, among others.
Pigments attenuate light by absorption and/or altering the direction of propagation through scattering and/or reflection. Thus, opaque pigments inherently reflect, absorb or otherwise interfere with the transmittance of ultraviolet light through a pigmented ink or coating, and, consequently, impede the penetration of the UV radiation into the lower layers of a pigmented film during curing. Opacity and color strength are, as a rule, desirable properties for inks and coatings. Full cure of UV curable inks and pigmented coatings is difficult because the pigments impede the penetration of radiation. A number of patents have been written to describe processes and formulations for improving the cure of these inks and pigmented coatings, for example, such as described by Muthiah, et. al. in U.S. Pat. No. 6,017,640. Here dual thermal and UV curable ingredients are incorporated into an opaquely pigmented powder coating formulation in order to fully cure through the entire thickness of a film. In U.S. Pat. No. 5,047,556, Kohler, et. al. describe the problem of curing pigmented systems that absorb a large proportion of the active wavelength so that higher concentrations of photoinitiators and/or sensitizers must be used. In order to help overcome this, photoinitiators which combine two basic structural types were developed in order to absorb UV wavelengths in a wider xe2x80x9cabsorption windowxe2x80x9d.
The present invention relates to a method of transfer printing. A coated media is printed with ink, the area of the media which is not covered with ink is cured by exposure to ultraviolet radiation, and the printed image is transferred to a final substrate.
The media is coated with a radiation curable, including electron beam or UV curable, coating. Upon exposure to electron beam or ultraviolet radiation, the coating in the exposed, non-imaged area cures, and becomes permanently bonded to the base sheet. The ink in the imaged area effectively blocks, absorbs and/or reflects the radiation, and does not allow polymerization under the imaged area. The image is then transferred to a final substrate by placing the image printed on the media in contact with the final substrate, followed by the application of energy, for example radiation, or heat and/or pressure. In this way, the image is permanently bonded to the final substrate, while no overprint, or non-imaged area, that may be visible and/or felt is transferred to the final substrate.
It is an object of the present invention to provide a media containing a radiation curable coating. It is another object of the invention to provide a radiation curable coating that receives a printed image. It is a further object of the invention to provide an image formed from an ink containing reactive ingredients that crosslink during the transfer step to form a permanent image from the above said media onto the final substrate. It is a still further object of the invention to provide a process for curing the radiation curable coating in the non-printed areas of the media. It is an object of the invention to provide a media that accepts a printed image, is radiation curable in the non-imaged area, and the imaged area is transferable to a final substrate. It is an object of the invention to provide a method for transferring an image to a heat- and/or pressure-sensitive final substrate without damage to the substrate. It is a further object of the invention to provide an image on a final substrate that is further cured with radiation.